(1) Field of the Invention
This invention pertains to the field of fans of the type mounted on shafts of electric motors and other dynamoelectric devices for cooling such devices during operation. More particularly, this invention pertains to a radial or centrifugal fan wherein the outer portion of its blades are configured to induce radial-flow on air expelled by the fan, and wherein the central portion of the fan""s blades are configured to induce partial axial-flow and partial radial-flow to improve the intake of air into the fan from one of the fan""s axially opposite sides. Additionally, the fan of the preferred embodiment has an annular wedge shaped ring connecting the trailing edges of the fan""s blades that acts as a diverter to channel air exiting the fan to opposite axial sides of an annular grill bar on a cover that encloses the fan. The configuration of the fan of the invention improves efficiency of the fan without increasing its size, thereby providing greater cooling capacity over conventional radial fans of the same size.
(2) Description of the Related Art
Many dynamoelectric devices such as appliance motors, hand tool motors, generators, and alternators utilize fans mounted on their rotor shafts to provide for air cooling of stator and rotor windings of the devices during their operation. Typically such fans are mounted at an axial end of the dynamoelectric devices immediately adjacent the device housing and are configured to pull or push air through the housing and between the rotor and stator.
The majority of dynamoelectric devices are generally cylindrical in shape and the fans are commonly configured to have nearly the same diameter. It is also common for such devices to have a cover enclosing the fan, or to place the fan within the housing of the devices, to prevent objects from contacting the fan blades. Additionally, it is generally desirable to configure cooling fans in a manner such that they take only a minimum of space, since such cooling fans must generally fit within a specific cylindrical space of minimal axial length.
Although some machines in which dynamoelectric devices operate allow for the use of axial-flow fans, the configurations of many machines in which dynamoelectric devices are used often necessitate the use of radial-flow fans which discharge air radially outward. Radial-flow fans are designed to obtain maximum air flow rates for a given configuration, unlike centrifugal compressors which are often designed to obtain large pressure differentials under low flow rate conditions. Other design considerations include costs and whether the fan must operate in opposite directions of rotation.
Perhaps the simplest radial-flow fan design is a straight blade fan. Straight blade fans utilize a plurality of blades extending radially from a central hub. Like other fan designs, the hub of a straight blade fan is typically a generally cylindrical body having a through-hole aligned with its center axis for mounting the fan to the shaft of a dynamoelectric device. It is also common for the through-hole to be keyed with the shaft to insure that the fan rotates with the shaft without slippage. The blades of a straight blade fan are typically flat rectangular members oriented parallel to the center axis of the hub such that air is forced through the fan purely by centrifugal force. Thus, straight blade fans are typically symmetric about a plane that is perpendicular to the center axis of the hub and act to draw air inward from both of the opposite axial sides of the fan in response to the fan blades pushing air radially outward from the center hub.
To reduce the amount of air drawn into a radial fan from the side of the fan that faces away from the dynamoelectric device to be cooled, such radial fans often have an imperforate disk shaped backing mounted for rotation with the fan on the side of the fan farthest from the housing of the dynamoelectric device. Such backings generally increase the amount of air drawn into the fan from the side of the fan facing the housing of the dynamoelectric device. However, such backings take up axial space and thereby reduce the axial width of the fan blades for a given total axial length of the fan, thereby decreasing the total output of the fans compared to non-backed fans of equal axial length. Additionally, backings also increase the amount of material required to manufacture such fans.
A preferred method of reducing the amount of air drawn into a radial fan from the side of the fan that faces away from the dynamoelectric device is to configure the cover which typically surrounds the fan with an imperforate disk shaped end that is positioned with an axial gap between it and the fan. Thus, the disk shaped end of the cover acts much as a backing does to increase the amount of air drawn into the fan from the side of the fan facing the housing of the dynamoelectric device, without reducing the axial width of the fan blades.
Straight blade radial fans have an advantage of operating equally well in either direction of rotation. For this reason, straight blade radial fans are often used to cool dynamoelectric devices whose shafts rotate in opposite directions during operation. However, for those devices whose shafts seldom or never rotate in opposite directions, straight blade fans need not be used and other configurations having blades that curve in a plane perpendicular to the center axis of the fan hub have been used. By curving the blades of a radial fan in a direction opposite that of the rotation (commonly called a backward curved radial fan), the fan exhausts air using both centrifugal force and force caused by the blade pushing the air in partially the radial direction. However, such curved blades may or may not increase the overall air output, since curving the blades also reduces the circumferential velocity of the air passing through the fan and thereby decreasess the centrifugal force component generated by the fan. Generally, straight blade radial fans produce a greater air flow rate than backward curved blade fans of the same size and are thus desirable for use with most dynamoelectric devices.
Although radial-flow fans have proven effective for cooling dynamoelectric devices, it remains advantageous to design fans having ever greater efficiency. Furthermore, it is desirable to increase the efficiency of such fans without increasing the size of the fans and without significantly increasing the cost or adding additional components to the dynamoelectric device assembly.
The radial fan of the present invention increases the flow rate of cooling air through a dynamoelectric device as compared to prior art straight blade radial fans of the same size. The increased airflow is a result of the configuration of the blades of the fan as well as the configuration of an annular ring joining the blades.
In general, the fan of the preferred embodiment of the invention is a fan configured for use with a dynamoelectric device whose rotor shaft rotates in only one direction. The fan of the preferred embodiment is made more efficient by configuring the blades to perform as a conventional radial fan near their trailing edges while also configuring the blades to perform as a mixed-flow fan nearer the shaft or inlet. This is achieved by splitting each of the blades into two sections and orienting the chord-line of the blades near the hub at an angle relative to the axis of the hub while maintaining the chord-line of the blades near the trailing edges of the blades parallel to the hub axis. By configuring the blades as a mixed-flow fan near the hub of the fan, the fan more efficiently draws air from its axial side facing the dynamoelectric device. This results in a corresponding higher radial air flow rate from the fan and greater cooling of the dynamoelectric device.
The annular ring of the preferred embodiment of the fan also increases the flow rate from the fan by channeling the air into exhaust openings of the cover that circumferentially surrounds the fan. The cover used with the preferred embodiment of the fan has an annular grill bar which surrounds the blades of the fan and axially separates pairs of openings in the cover to prevent objects and fingers from contacting the blades of the fan when the fan is rotating. Thus, the grill bar of the cover partially obstructs the flow of air from the fan and the air must pass on either axial sides of the grill bar as it is being exhausted. The ring of the preferred embodiment has axially opposite sides that taper toward each other as they extend radially inward. As air flows past the ring while being exhausted, the taper of the ring axially separates the flow in a streamline manner, which then allows the air to pass the grill bar of the shroud more efficiently.
Like the preferred embodiment of the fan, an alternative embodiment of the fan has blades configured such that the chord-line of the blades near the hub are at an angle relative to the axis of the hub while the chord-line of the blades near the trailing edges of the blades are parallel to the hub axis. However, the disclosed alternative embodiment of the fan achieves this by extending a portion of the each of the blades, near the hub, axially forward of the remainder of the blade and curving such portions so that the portions are rotationally in advance of the remainder of the blades. Additionally, the alternative embodiment of the fan utilizes a ring positioned at the root edge of each of the blades that acts similar to a backing but that is absent where the chord-line of each blade is oriented at an angle relative to the axis of the hub. This allows the fan to be manufactured using simple molding methods that would not be possible if the fan had a disked shaped backing rather than a ring.
While the principle advantages and features of the invention have been described above, a more complete and thorough understanding of the invention may be attained by referring to the drawings and the detailed description of the preferred embodiment, which follow.